While I doubt malicious intent on FB's part there's certainly a financial incentive to ignore this problem. How would their earnings reports change if they stopped charging at least some advertisers for 80% of clicks?
Advertisers monitor to see performance on the other end, and when the campaigns don't yield actual success they move on.
The only real exception to this are mega-brands that advertise just to build namespace. Everyone else expects direct action from some hefty percentage of their ads.
Imagine a 100 person company with 5 people dedicated to advertising. Leadership says to increase on-line presence. Some recent college grad on the advertising team buys FB ads. Next month leadership is pleased that they're advertising on Facebook. Also leadership is pleased that sales increased x%. The FB ads could be completely worthless. Some non-trivial percentage of clicks could be from bots and no one would notice.
Most ads target a specific outcome: You buy a product, create an account, joining a mailing list, etc. Even the most technically incompetent advertisers establishes these fundamental metrics such that they can say "I spent $1,000,000 on Facebook ads and only got ten signups. I spent $1,000,000 on iAds ads and got ten million signups." That sort of thing.